Stainless steel or corrosion-resistant steel is a kind of metallic alloy that is found in a variety of forms. It serves our practical needs so well that it is difficult to find any sphere of our life, where we do not use this type of steel. We use this steel to make architectural structures, automobiles, home appliances, kitchen appliances and so many other products. It is so widely used on account of its strong resistance towards corrosion that can be caused due to continuous changes in atmospheric conditions. It is rarely affected by changes and extremely high or extremely low temperatures. There are many other distinguishing properties that make it useful. Such attributes are due to its unique composition.
The major components of stainless steel are iron, chromium, carbon, nickel, molybdenum, and small quantities of other metals. On the basis of its crystalline structure, stainless steel can be broadly categorized into four different types. It is to be noted that iron is always the main constituent, while the rest of the chemical substances are present in varying proportions in each type. However, in any of them, the chromium content should not go below 11 percent. This is because, the rich content of chromium in stainless steel makes it highly corrosion-resistant. Here, their chemical composition has been described in detail.
More than seventy percent of the total stainless steel produced belongs to this category. It has at least 16 percent of chromium, 0.15 percent of carbon, and a good amount of nickel and manganese along with iron. Such a composition enables it to maintain its distinct structure at any given temperature, from the cryogenic to its melting point. It is used in various cryogenic applications due to presence of nickel that prevents the brittleness of steel. Sometimes, elements like molybdenum or titanium are added in small amounts to bring about changes in certain properties to make it suitable for any specific application that requires corrosion resistance at extremely-high temperatures. Usually, this steel does not show any magnetic properties.
This variety is mostly made of chromium, molybdenum, and small amounts of nickel, carbon aluminum, and titanium. Some of the types also include lead as a component. The chromium content lies between 10.5 to 27 percent. The main characteristics of these steels is that they have the power to resist corrosion even at high temperatures. This is possible on account of the large quantity of chromium present in it. However, its durability is comparatively lesser than austenitic steel. This type of steel possesses magnetic properties.
This can be described as a mixture of equal quantities of austenite and ferrite stainless steels. When used for commercial purposes, these elements can at times be in the ratio of 40:60. Hence, it has higher strength and resistance against pitting, stress corrosion, and cracking. Both chromium and molybdenum are present in higher quantity in this type of steel as compared to austenitic stainless steel. The chromium content varies between 19 to 28 percent, and molybdenum can go up to 5 percent. It has lesser amounts of nickel than those found in austenite.
This type is made of 12 to 14 percent of chromium, about 1 percent each of carbon and molybdenum and about 2 percent of nickel. Due to lower-chromium content, it is more brittle than other stainless steels. It is a very tough and strong kind of steel due to the high amounts of carbon. It can be used in a wide range of applications because of its strength, but is not very effective in resisting corrosion.
Today, due to advancement in the technology of steel making, steel manufacturers can control the physical and chemical properties of stainless steel according to requirement. This can be obtained by altering the quantity of the various chemical substances that are used in stainless steel, which in turn will change its quality to suit the need of a particular purpose.